33 GHz Interferometer

This picture shows the 33 GHz CMB interferometer inside its enclosure. The frame inside the enclosure has been clad with aluminium sheeting to reduce the level of ground pick-up.

Location

Teide Observatory

Type

2-element interferometer

Scanning

Earth rotation (drift)

Frequency

32.5 GHz

Bandwidth

3 GHz

Primary beam

5° × 3° FWHM

l-range

109 ± 24, 212 ± 22

The 33 GHz interferometer operates according to the same principals as the 5 GHz CMB interferometer at Jodrell Bank. It has two horns, but unlike the radiometers they both point in the same direction. Behind the horns is the cryostat containing two low noise receivers, cooled to approximately 15 K with a helium expansion system. Signals from the receivers are fed to a complex correlator. This isshown in the picture sitting on the chair, undergoing tests. It produces cosine and sine fringes as the antennas sweep across a source. The fringes are converted to digital form and sent to the logger computer.

The schematic gives a cut-away view of the telescope, showing the antennas with the cryostat behind. The cryostat and correlator are connected by a pair of flexible cables. One of these is split, with an adjustable section inserted, to allow for equalization of the lengths. The air inside the telescope and various chassis plates are carefully temperature-controlled, to avoid temperature-sensitivity problems with the electronics.

The other Tenerife CMB telescopes are mounted horizontally, with a mirror collecting signals from the chosen strip of sky. However, this instrument is different. In this case the two mirrors form an integral part of the antenna system, so the whole receiver unit is tipped in elevation, supported by a tripod on either side. In the schematic the elevation axis is indicated by the red circle.

With the 33 GHz interferometer we are searching for structure in the CMB on angular scales around 1 to 2 degrees. Data from the interferometer are processed with SJM MIDAS.